All pictures taken on film exhibit some degree of “graininess” because of the discrete crystals of silver halide in the photographic emulsion. In still image photography, this graininess may be imperceptible in a typical 3″×5″ print. In motion picture imaging, the random nature of the grain structure will cause successive frames of the same scene to “sparkle” or “boil”. Aggravating the problem is the fact that motion pictures are displayed with high magnifications making graininess more visible for theatre-goers seated closer to the screen.
Small film areas (e.g. 8 mm, 16 mm) require more magnification to fill a movie screen thus magnifying the grains. Moreover, noise accumulates with each intermediate stage of film transfer as the image chain progresses from capture to release print.
Thanks to the advent of digital image processing, it is possible to design filters to reduce noise in a digital image by sampling the same image across multiple frames. Many modem movies involve some form of digital intermediate where the image is digitized using a film scanner. The digital intermediate stage is required for inserting computer graphics into live action shots and for a variety of special effects. The pixels in every frame of a movie can be altered before being printed back to film.
During the 1990's, motion imaging digital effects were tailored to match the quality of traditional 35-mm film processes. Subsequently, digital effects processes expanded into long form digital intermediate. This allows high speed processing of an entire feature film, while preserving the quality and look of the optical film process. Systems analyses indicate that we have the opportunity to distribute to theaters film or digital images that provide a significant increase in overall theatrical image quality by introducing a new generation of digital intermediate. This preferred approach uses current 35-mm film, and involves modest changes in production costs and theater equipment.
Consequently, by exploiting advances in digital technology, using new algorithms and high-speed high-resolution scanning and recording we can give film a clearly superior look. In fact, it will become possible to provide at least 70 mm quality by digitally processing images originated in a 35 mm format. Similarly pre-compensating for artifacts and losses in digital electronic projectors can be achieved in postproduction.
Well-known techniques for filter design can be applied to filtering of motion picture sequences. The difficulty lies in designing a filter that leaves the desired image content intact while removing the undesirable noise. As there is normally some degree of “art” to filtering because the process can change the “look” of the image, what is needed is a technique for adjusting the filters to emulate a desired grain response, such as that of larger film formats.